Catalytic platform for co2 reduction

ABSTRACT

Carbon dioxide reduction includes bonding CO 2  with an activator so as to form an activated intermediate. An electrical potential is applied to the intermediate so as to cause reduction of the CO 2  in the intermediate. The CO 2  reduction generates an organic fuel and releases the activator from the intermediate. The use of the intermediate suppresses H+ reduction reactions that compete with generation of the desired fuel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/930,247, filed on Jan. 22, 2014, andincorporated herein in its entirety; and this application also claimsthe benefit of U.S. Provisional Patent Application Ser. No. 62/051,208,filed on Sep. 16, 2014, and incorporated herein in its entirety; andthis application also claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/938,497, filed on Feb. 11, 2014 and incorporatedherein in its entirety; and this application also claims the benefit ofU.S. Provisional Patent Application Ser. No. 62/051,644, filed on Sep.17, 2014 and incorporated herein in its entirety; and this applicationalso claims the benefit of U.S. Provisional Patent Application Ser. No.62/051,789, filed on Sep. 17, 2014 and incorporated herein in itsentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support underDE-SC0004993/T-106808 awarded by the Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to solar fuels generation, and moreparticularly, to reduction of chemical components for solar fuelsgeneration.

BACKGROUND

Solar fuels generators can be used to convert sunlight into a fuel thatcan be stored for later use. One example of a solar fuel reduces CO₂ soas to generate an organic fuel. However, there are multiple differentfuels that can be generated from CO₂ reduction. Examples of organicfuels that can be generated from CO₂ reduction include methane,methanol, ethanol, propanol, butanol, and glucose. During CO₂ reduction,the chemical reactions that generate these different fuels compete withone another. As a result, it is difficult to generate a particularorganic fuel from CO₂ reduction. For instance, it is often desirable togenerate methane from CO₂ reduction. However, the methane is oftenchemically generated at elevated temperatures. At the temperaturesincrease, the selectivity of for reducing CO₂ to methane decreasesmaking methane generation even less efficient. Further, the generationof these fuels from CO₂ reduction is associated with high overpotentialsand kinetic barriers. As a result, there is a need for a platform thatallows efficient conversion of CO₂ to a particular one of the fuels.

SUMMARY

Carbon dioxide is bonded with an activator so as to form anintermediate. An electrical potential is applied to the intermediate soas to reduce the CO₂ in the intermediate. The CO₂ is reduced such thatan organic fuel is generated and the activator is released from theintermediate.

An electrode at which the CO₂ is reduced can have an active layer on anelectrode base. The active layer includes a polymer that includes one ormore reaction components selected from a group consisting of a CO₂reduction catalyst and the activator. The electrode can be included in aCO₂ reduction device such as a solar fuels generator or an electrolysisdevice.

The disclosure provides a method of CO₂ reduction, comprising: bondingCO₂ with an amine activator so as to form an intermediate; and applyingan electrical potential to the intermediate under conditions that causereduction of the CO₂ bound in the intermediate. In one embodiment, thereduction of the CO₂ in the intermediate generates a liquid organicfuel. In one embodiment, the reduction of the CO₂ in the intermediategenerates a gaseous organic fuel. In one embodiment, the the reductionof the CO₂ generates methane. In one embodiment, the CO₂ is linearbefore bonding with the activator but has a bent configuration in theintermediate. In one embodiment, the intermediate is a carbamate orcarbamic acid. In a further embodiment, the intermediate is N-carbamateamine-CO₂. In one embodiment, the activator is released from theintermediate upon reduction of the CO₂ in the intermediate. In oneembodiment, the CO₂ is bonded to the activator in a CO₂ scrubber.

The disclosure also provides a CO₂ reduction device, comprising: anamine activator that bonds with CO₂ so as to form an intermediate; andelectrodes that apply an electrical potential to the intermediate underconditions that cause reduction of the CO₂ in the intermediate. In oneembodiment, the reduction of the CO₂ in the intermediate generatesmethane. In one embodiment, the the CO₂ is linear before bonding withthe activator and has a bent configuration in the intermediate.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates an electrolysis system as an example of a CO₂reduction device.

FIG. 2 is a perspective view of an electrode having an active layer onan electrode base.

FIG. 3A and FIG. 3B illustrate an electrode having a patterned activelayer. FIG. 3A is a perspective view of the electrode.

FIG. 3B is a cross section of the electrode shown in FIG. 3A taken alongthe line labeled B in FIG. 3A.

FIG. 4A illustrates a precursor for a sidechain that includes acatalyst.

FIG. 4B illustrates a precursor for a sidechain that includes anactivator.

FIG. 5 illustrates the precursors for a polymer to be included in theactive layer of an electrode.

FIG. 6A and FIG. 6B illustrates a solar fuels generator that includes anelectrode constructed according to FIG. 2 or FIG. 3A through FIG. 3B.FIG. 6A is a cross section of the solar fuels generator.

FIG. 6B is a sideview of the solar fuels generator shown in FIG. 6Ataken looking in the direction of the arrow labeled V in FIG. 6A.

FIG. 7 illustrates cyclic voltammograms for a variety of differentelectrolytes.

DETAILED DESCRIPTION

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which the invention pertains. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice for testing of the invention(s), specific examples ofappropriate materials and methods are described herein.

Also, the use of “or” means “and/or” unless stated otherwise. Similarly,“comprise,” “comprises,” “comprising” “include,” “includes,” and“including” are interchangeable and not intended to be limiting.

It is to be further understood that where descriptions of variousembodiments use the term “comprising,” those skilled in the art wouldunderstand that in some specific instances, an embodiment can bealternatively described using language “consisting essentially of” or“consisting of.”

All publications mentioned herein are incorporated herein by referencein full for the purpose of describing and disclosing the methodologies,which are described in the publications, which might be used inconnection with the description herein. The publications discussed aboveand throughout the text are provided solely for their disclosure priorto the filing date of the present application. Nothing herein is to beconstrued as an admission that the inventors are not entitled toantedate such disclosure by virtue of prior disclosure.

Carbon dioxide is generally a linear compound that can coordinate withan activator so as to form an intermediate having the CO₂ in a bentconfiguration. An electrical potential is then applied to theintermediate under conditions that cause the CO₂ to be reduced andreleased from the activator as a reduced fuel such that the activator isrecovered. The CO₂ reduction generates an organic fuel that depends onthe selection of the activator. For instance, an activator can beselected that results in the generation of methane gas. The use of theactivator in the CO₂ reduction is surprisingly and unexpectedlyselective for the desired fuel. For instance, organic products thatresult from competing CO₂ reduction reactions are not substantiallygenerated. Further, hydrogen from the competing hydrogen generatingreactions is also not substantially generated. Without being bound totheory, this result may be a function of the bent CO₂ causing theactivation energy of the CO₂ reduction to drop far enough below theactivation energy of the competing reactions that the targeted reactionbecomes dominant. Additionally or alternately, the reduction of the CO₂included in the intermediate may occur by a different mechanism thatexcludes competing reactions.

The activators can be used in CO₂ reduction devices such as solar fuelsgenerators and electrolysis systems. These devices can operate on aliquid electrolyte in which the CO₂ is dissolved. The fuel that isgenerated in these devices can be a gas such as methane. Since thegaseous fuel is formed in the liquid electrolyte, the fuel can formbubbles that exit from the liquid electrolyte. As a result, the processof separating the product from the other system components issimplified.

The activator can be included in a liquid electrolyte along with otherreaction components such as CO₂ reduction catalysts. Additionally oralternately, a CO₂ reduction device can include one or more CO₂reduction electrodes that include an active layer having a polymericmedium. The polymeric medium can include a polymer that constrains theactivator and/or CO₂ a reduction catalyst. The active layers provide aplatform for constraining the activator and/or catalyst near the surfaceof the electrode at which the CO₂ reduction occurs. Further, the activelayer can protect the underlying portions of the electrode from anelectrolyte in which the electrode is positioned. As an example theactive layer can protect the underlying portions of the electrode fromhighly acidic electrolytes. Additionally or alternately, the activelayer can protect the underlying electrode from competing reactions suchas 0₂ reduction and/or can protect the electrode from highly acidicenvironments. Accordingly, the active layer can increase the servicelife of the electrode.

Additionally, the disclosed CO₂ reduction technology may have surprisingenvironmental benefits. Carbenes and amines are examples of materialsthat may be suitable for use as activators. Many of these materials arecurrently used to bond carbon dioxide in CO₂ scrubbers. Once CO₂ isbonded to these materials in scrubbing technology, the resulting productis considered waste that must be destroyed or stored. However, thiswaste product may be a suitable intermediate for use in the disclosedCO₂ reduction. As a result, the disclosed technology can provide anapplication for these waste products and may also provide a method ofrecovering the original material for use in a scrubber.

As noted above, CO₂ reduction can be used to generate a variety ofdifferent hydrocarbon fuels. The reaction pathways associated with thereduction of CO₂ can be represented by the following generalizedreaction: mCO₂+n H₂O→C_(m)H₂nO_((2m+N−2P))+p O₂ where m, n, and p arenon-negative numbers and, in some instances, are integers;C_(m)H₂NO_((2M+N−2P)) represents the fuel produced in this reaction andCO₂ serves as the reactant that is delivered to the photocathodes. Thisreaction illustrates that in some instances, the organic fuels generatedby CO₂ reduction include or consist of carbon, hydrogen, and, in someinstances oxygen. For instance, this reaction can produce fuels such asmethanol, methane, ethanol, formic acid, acetic acid, ethanol, propanol,1,3 propanediol, 2-oxybutyric acid, butanol, and glucose. Accordingly,the generated fuels can be a gas such as methane or a liquid such asethanol. When trying to use this reaction to generate a particular fuel,the reactions that generate different fuels compete with one another.For instance, the reaction can be used to generate methane gas (i.e.,m=1, n=2, p=2). When using an electrolysis device, the reaction at thecathode during the generation of methane can be illustrated as:CO₂+8H⁺+8e⁻→CH₄+2H₂O; however, the reactions that generate other fuelswill compete with the generation of methane. Further, hydrogen gasgeneration will also compete with the methane generation. A chemicalplatform is presented for selectively targeting the generation ofparticular fuels from CO₂ reduction.

The CO₂ reduction is performed in the presence of various reactioncomponents. Examples of reaction components include catalysts andactivators. Catalysts can be used to reduce the activation energyassociated with CO₂ reduction. CO₂ is a linear molecule; however,activators can coordinate with the CO₂ so as to bend the CO₂ and/or makethe CO₂ non-linear. For instance, an activator can bond with the CO₂ soas to form a CO₂ adduct with the CO₂ in a non-linear configuration. Insome instances, the activator causes the CO₂ to be bent at an anglegreater than 110°. The CO₂ adduct can act as an intermediate compound inthe reduction of CO₂. In some instances, the activators arepre-activators in that the activator coordinates with the CO₂ before theCO₂ is subject to catalytic activity.

In some instances, the carbon in CO₂ covalently bonds with theactivator. Carboxylates and carboxylic acids are examples of CO₂ adductsthat can be formed from CO₂ and an activator. As an example, the resultof the activator bonding with the CO₂ can be a carboxylate where the CO₂takes on a non-linear configuration such as

In some instances, the intermediate is zwitterionic in that theintermediate is neutral but has one or more positive charges and one ormore negative charges.

Since CO₂ is an electrophile, nucleophiles can serve as the activator.An example of suitable activator includes carbenes such as acycliccarbenes and cyclic carbenes. Example carbenes include, but not are notlimited to, imidazoles and imidazole-based carbenes. The carbene can bea N-heterocyclic carbene. An example N-heterocyclic carbene can berepresented by

where R represents a hydrogen, halogen, or an organic moiety and R′represents a hydrogen, halogen, or an organic moiety. For instance, theN-heterocyclic carbene can be 1,3-bis(2,6-diisopropylphenyl)imidazolium.The N-heterocyclic carbene can bond with CO₂ to from zwitterionicimidozolium carboxylates. An example zwitterionic imidozoliumcaboxylates can be represented by

For instance, the zwitterionic imidozolium caboxylates can be1,3-bis(2,6-diisopropylphenyl)imidazolium carboxylate.

Other examples of activators include, but are not limited to, aminessuch as ethanolamine, 2-amino-2-methyl-1-propanol, diethanolamine,piperazine, methyldiethanolamine, diisopropanolamine,2-(2-aminoethoxy)ethanol. These materials bond with CO₂ to formcarbamates and/or carboxylic acids that include the CO₂ in a bent ornon-linear configuration. For instance, these materials can bond withCO₂ to form N-carbamate amine-CO₂ adducts. As an example, an amine canbe represented by

where R represents a hydrogen, halogen, or an organic moiety and R′represents a hydrogen, halogen, or an organic moiety, and at least oneof R and R′ represents an organic moiety. The amine can bind the CO₂ soas to form a carbamate represented by

The hydrogen in the amine serves as the source of the hydrogenillustrated in the carbamate.

Other examples of suitable activators include, but are not limited to,hydroxide (OH⁻) and alkoxides (RO⁻). These activators also bind and bendCO₂. For instance, hydroxide anions react with carbon dioxide in asimilar fashion to amines and N-heterocyclic carbenes to formbicarbonate anions intermediates such as

The use of hydroxide as an activator may of particular interest in thefield of solar fuels, as several of the proposed solar-fuel devicesinvolve aqueous conditions in basic media due to limitations on theanodic side chemistry of a solar-fuel device. Basic alkoxide anionsreact with carbon dioxide in a similar fashion to amines andN-heterocyclic carbenes to form alkylcarbonate anion intermediates suchas

The use of hydroxide as an activator may be particular interest in thefield of solar fuels, as several of the proposed solar-fuel devicesinvolve aqueous conditions in basic media, which may contain alkoxidesas bases to preactivate and bind CO₂ to form organic carbonates.

Another example of a suitable class of activators includesN-Heterocyclic Olefins (NHO) such as

where R represents a hydrogen, halogen, or an organic moiety and R′represents a hydrogen, halogen, or an organic moiety. N-HeterocyclicOlefins have been shown to be sufficiently Lewis-basic to react and bendCO₂ to form CO₂ adduct intermediates such as

Other examples of suitable activators include nucleophilic bases thatbond CO₂ in a bent configuration.

The activator and CO₂ can be dissolved in a liquid electrolyte. When theactivator is a carbene, suitable solvents for the electrolyte include,but are not limited to, organic solvents such as methylene chloride,dimethylformamide, acetonitrile, tetrahydrofuran, and benzonitrile andmixtures of these solvents. In some instances, water can serve as asolvent for a carbene activator or can be included in the solvent. Whenthe activator is an amine, suitable solvents for the electrolyteinclude, but are not limited to, water, ionic liquids, methylenechloride, dimethylformamide, acetonitrile, tetrahydrofuran, andbenzonitrile and mixtures thereof. Amines are generally more soluble inwater than carbenes and accordingly provide an improved opportunity forthe use of aqueous electrolytes.

The activator and solvent can be selected such that the CO₂ andactivator spontaneously form the intermediate in the electrolyte. Forinstance, carbenes such as the N-heterocyclic carbene represented by

can react spontaneously in the liquid electrolyte to form thecarboxylate intermediate. N-carbamate amine-CO₂ adducts formspontaneously upon exposure of the amine to CO₂ in a liquid electrolyteor in a gas.

As will be discussed in more detail below, the activator represented by

is highly selective for the generation of methane gas; however, otheractivators will be selective for other fuels. Accordingly, the activatorcan be selected so as to generate the desired fuel.

Other reaction components include, but are not limited to, catalystssuch as CO₂ reduction catalysts. Example CO₂ reduction catalystsinclude, but are not limited to, metals such as Fe, Co, Ni, Cu, Ag, Au,Sn, Mo, Ir, Pt, Ru, Ti, Zr, Ta, Mg, Li, Hg, Al and Zn. CO₂ reductioncatalysts can include these metals in coordination complexes such asorganometallics. Organometallics are coordination complexes where theligands are organic and/or “organic-like” as in the case of ligands suchas phosphines, hydride, and CO. In some instances, the organometalliccatalysts have organic ligands. As an example, ferrocene is anorganometallic material that includes iron and serves as a CO₂ reductioncatalyst. Examples of organometallic materials that can functions as CO₂reduction catalysts include, but are not limited to, [Co^(III)N₄H(Br)₂]⁺whereN₄H=2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(7),2,11,13,15-pentaene,Mn(bpy-Bu)(CO)₃Br where bpy represents 2,2′ bipyridine, Re(bpy)(CO)₃Brwhere bpy represents 2,2′ bipyridine, [Ni(cyclam)]²⁻ where cyclamrepresents 1,4,8,11-tetraazatetracyclodecane, [Co^(I)L]⁺ where Lrepresents5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene;[Ru(bpy)₂(CO)₂]²⁺ where bpy represents 2,2′ bipyridine, TPPFeCl whereTPP represents tetraphenylporphyrin, FeTDHPPCl where TDHPP represents5,10,15,20-tetrakis(2′,6′-dihydroxyphenyl)-porphyrin, FeTDMPPCl whereTDMPP represents 5,10,15,20-tetrakis(2′,6′-dimethoxyphenyl)-porphyrin;CoTPP where TPP represents tetraphenylporphyrin; [(η⁵-Me₅C₅)M(bpy) Cl]M=Ir, Rh where bpy represents 2,2′ bipyridine, [Pd₂ (CH₃CN)₂(eHTP)](BF₄)₂ where eHTP represents [(Et₂PCH₂CH₂)₂PCH₂P(CH₂CH₂PEt₂)₂],and Pd (triphosphine) (CH₃CN)](BF₄)₂.

One system for the generation of hydrocarbon fuels is an electrolysissystem. FIG. 1 provides an example of an electrolysis operated as a CO₂reduction device. The system includes a vessel 10 having a reservoir.Anodes 14 and cathodes 16 are positioned in the reservoir such thatanodes 14 and cathodes 16 alternate with one another. The anodes 14 andcathodes 16 are parallel or substantially parallel with one another. Theelectrolyte 18 is positioned in the reservoir such that anodes 14 andthe cathodes 16 are in contact with the electrolyte 18. Suitable anodesinclude, but are not limited to, fluorine doped tin oxide film on glass,a titanium foil current collector, Fe, Co, Ni, Cu, Ag, Au, Sn, Mo, Ir,Ru, Pt, Ti, Zr, Ta, Mg, Li, Hg, Al and Zn and their respective oxides.Suitable cathodes include, but are not limited to, glassy carbon, carbonfoam, reticulated vitreous carbon, carbon cloth, carbon felt or Fe, Co,Ni, Cu, Ag, Au, Sn, Mo, Ir, Ru, Pt, Ti, Zr, Ta, Mg, Li, Hg, Al and Zn.Alternative cathode constructions are disclosed below. The cathode andanode are connected to a voltage source 20 that is sufficient to applythe overpotential needed to cause the illustrated electrolysis. Thevoltage source can be any voltage source such as a photovoltaic voltagesource, battery or other electronics.

The CO₂ is dissolved in the electrolyte. Additionally, one or more ofthe reaction components can be included in the electrolyte. Forinstance, one or more activators and/or one or more catalysts can bedissolved in the electrolyte solvent. Alternately, the one or more ofthe reaction components can be confined or substantially confined at thesurface of the cathode. For instance, one or more activators and/or oneor more catalysts can be confined at the surface of the cathode.Alternately, one or more reaction components can be confined orsubstantially confined at the surface of the cathode and one or moreother reaction components can be dissolved in the electrolyte. Forinstance, one or more activators can be dissolved in the electrolytesolvent and one or more catalysts can be confined at the surface of thecathode.

During operation of the electrolysis system, the voltage source 20generates a voltage between the one or more anodes and the one or morecathodes so as to drive an electrical current through the electrolyte.The resulting electrical potentials drive the illustrated reactions. Forthe purposes of illustration, FIG. 1 illustrates the electrolysis systemgenerating CH₄ in an aqueous electrolyte. The CO₂ dissolved in theelectrolyte reacts with the activator so as to form an intermediate suchas a CO₂ adduct. As noted above, in some instance, the activator isselected such that the coordination of the CO₂ with the activator isspontaneous and reversible in the electrolyte. At the anode, water isoxidized so as to generate protons that are transported through theelectrolyte to the cathode. At the cathode, the intermediate, electronsfrom the cathode and protons from the electrolyte combine so as togenerate the methane. The activator is released from the intermediate inresponse to the reduction on the CO₂ in the intermediate. The methane isgenerated as a gas and can form bubbles in the electrolyte. The methanegas can exit from the electrolyte and enter the atmosphere over theelectrolyte.

Although FIG. 1 illustrates the electrolysis system used to generatemethane, the electrolysis system can be used to generate other fuels bychanging the activator. Additionally, water is illustrated as the protonsource in the electrolyte, however, when the electrolyte includes one ormore organic solvents, a proton source can be added to the electrolyte.Suitable proton sources include, but are not limited to, methanol,water, protic ionic liquids confined at the electrode or in liquidphase, protonated amines, mineral acids and mixtures thereof. One ormore proton source can be added to an aqueous electrolyte.

A variety of electrolysis experiments illustrate the fuel selectivitythat can be achieved through the use of activators. The electrolysisexperiments were performed using a variety of different liquidelectrolytes that included organic solvents. The electrolytes eachincluded at least two components selected from the group consisting ofcatalyst, intermediate, proton source, and CO₂. The intermediate wasbased on an activator selected to generate methane. The results arepresented in the following Table 1. A voltage of −1.5V was applied tothe electrolytes during the electrolysis experiments. The contents ofthe gaseous volume above the electrolyte were analyzed to identify theproducts produced, the faradic efficiency of production, and the molesof product produced. The first and second electrolytes both included theintermediate and the catalyst and both produced methane at Faradaicefficiencies of greater than 90%. The remaining catalysts did notinclude both the catalyst and activator and either failed to producemethane at all or produced methane at very low levels. Electrolytes thatexcluded the activator failed to produce methane. Further, some of theelectrolytes produced hydrogen gas much more efficiently than methane.

TABLE 1 Charge Faradaic Moles of Electrolyte passed Product efficiencyproduct 1. Catalyst, intermediate, proton 29 ± 6 C CH₄ 91 ± 4%  3.3 ×10⁻⁵ source, and CO₂ 2. Catalyst, intermediate, proton 27 ± 5 C CH₄ 93 ±2%  3.2 × 10⁻⁵ source, and N₂ 3. Intermediate, proton source, and N₂  8± 5 C H₂:CH₄ (12:1) 80 ± 7%  4.1 × 10⁻⁶:7.2 × 10⁻⁶ 4. Catalyst,intermediate, and N₂ 10 ± 7 C CO 78 ± 10% 4.0 × 10⁻⁵ 5. Catalyst, protonsource, and N₂  9 ± 7 C H₂, CO 72 ± 12% 1 × 10⁻⁵: 2.2 × 10⁻⁵ 6.Catalyst, proton source, and N₂  5 ± 2 C n/a n/a n/a

FIG. 2 is a perspective view of an electrode that includes an activelayer 22 on an electrode base 24. The illustrated electrode is suitablefor use as a cathode in the electrolysis system disclosed above. Theactive layer 22 includes, consists of, or consists essentially of apolymeric medium that confines one or more reaction components. Forinstance, the polymeric medium can include one or more polymers that arebonded to the activator and/or CO₂ reduction catalyst. Alternately, theactivator and/or CO₂ reduction catalyst can be entrapped within thepolymeric medium without being covalently bonded to a polymer in thepolymeric medium. In some instances, all or a portion of the one or morereaction components are covalently bonded to a polymer within thepolymeric medium. Accordingly, the polymeric medium can include the oneor more reaction components. Examples of reaction components include,but are not limited to, catalysts and activators.

The active layer 22 is immobilized on the electrode base 24. Forinstance, the polymeric medium can be bonded directly to the electrodebase 24. Alternately, the active layer 22 is immobilized on theelectrode base 24 through other means such as physisorption. In someinstances, the active layer 22 is immobilized on the electrode base 24as a result of the polymeric medium being covalently bonded to theelectrode base 24.

The electrode base 24 represents the portion of the electrode having atraditional electrode construction. Alternately, the electrode base 24represents the portion of the electrode having a traditional electrodeconstruction but with a prior active layer removed from the electrode.Accordingly, the active layer 22 can be added to a prior art electrodeor can replace an active layer 22 on a prior art electrode.

The electrode base 24 can be a current collector such as a metal foil orsheet, mesh, or conducting fabric. As will become evident from the belowillustration of a solar fuels generator, the electrode base 24 can be orinclude a semiconductor layer. Although the electrode base 24 is shownas a single layer of material, the electrode base 24 can includemultiple layers of material. For instance, the electrode base 24 caninclude one or more layers of active material on a current collector. Ininstance where the electrode is employed to reduce CO₂, the electrodeoperates as a cathode. During fabrication of the electrode, in someinstances, the electrode base serves as the support for the catalystlayer. Accordingly, the catalyst layer can optionally be formed directlyon the electrode base.

Although the active layer 22 is illustrated as being located on one sideof the electrode base 24, the active layer 22 can be located on bothsides of the electrode base. Additionally or alternately, FIG. 2illustrates the active layer 22 being located on the electrode base 24;however, the electrode base 24 can be embedded in the active layer 22.For instance, electrode base 24 can be a mesh that is embedded in theactive layer 22.

Although FIG. 2 illustrates the active layer 22 as a continuous layer ofmaterial, the active layer 22 can be patterned. For instance, recessesthat extend into the active layer 22 can define the pattern in theactive layer 22. The recesses can be channels in or through the activelayer 22, openings in or through the active layer 22. In some instances,the recessed form regions of the active layer 22 that are not continuouswith one another such as would occur when the active layer 22 isarranged in islands on the electrode base. The pattern can be a regularor periodic pattern or can be a random pattern. A medium can be locatedin the recesses. The medium can include, consists of, or consistessentially of all or a portion of the one or more reaction components.As an example, FIG. 3A and FIG. 3B illustrate an electrode whereopenings extend through the active layer 22. FIG. 3A is a perspectiveview of the electrode. FIG. 3B is a cross section of the electrode shownin FIG. 3A taken along the line labeled B in FIG. 3A. A medium 28 islocated in the recesses. The medium includes, consists of, or consistsessentially of all or a portion of the one or more reaction components.For instance, the medium 28 can include, consist of, or consistessentially of all or a portion of the one or more catalysts included inthe active layer 22 and/or the one or more activators included in theactive layer 22. The polymeric medium can exclude any reactioncomponents and all of the reactions components can be located in themedium. Alternately, the polymeric medium can confine a portion of thereaction components and the medium can include, consist of, or consistessentially of a different portion of the reaction components. As anexample, the polymeric medium can include a catalyst and the medium caninclude, consist of, or consist essentially of one or more activators.When the medium and the polymeric medium each includes one or morecatalysts, the one or more catalysts in the medium and the one or morecatalysts in the polymeric medium can be the same or different. When themedium and the polymeric medium each includes one or more activators,the one or more activators in the medium and the one or more activatorsin the polymeric medium can be the same or different. Suitable methodsfor placing the medium 28 in the recesses that define the patterninclude, but are not limited to, electroplating, electrolysis, surfaceinitiated polymerization, thermal deposition, plasma deposition,chemical vapor deposition, atomic layer deposition. The medium in therecesses can include components other than catalysts and activators. Forinstance, the medium in the recesses can include binders, diluents,conductors, optical scattering elements, stabilizers, membranes,barriers, reagents, fluidic channels.

During operation, the electrode is typically in physical contact withthe electrolyte. The electrolyte can be a solid or a liquid. When theelectrolyte is a liquid, the electrolyte can be absorbed in the activelayer 22. For instance, the polymeric medium can be hygroscopic and theelectrolyte can be absorbed and/or adsorbed by the polymeric medium. Insome instances, the absorption and/or adsorption causes swelling of theactive layer 22 and causes the active layer 22 to be a gel phase. Asuitable thickness for the active layer 22 includes, but is not limitedto, a thickness less than 1 micron, 100 nanometers, 10 nanometers and/orgreater than 1 nanometer, 10 nanometers, 100 nanometers.

During operation of the electrode, electrons from the electrode base 24interact with components in the active layer 22 and/or in theelectrolyte. The active layer 22 can be electrically conductive but neednot be. When the active layer 22 is electrically conductive, the activelayer 22 readily conducts the electrons to the components in the activelayer 22 and/or in the electrolyte. When the active layer 22 is notelectrically conductive the active layer 22 can be thin enough to permittunneling. For instance, the active layer 22 can be thin enough topermit tunneling of the electrons to the components in the active layer22 and/or in the electrolyte. When tunneling is desired in the activelayer 22, a suitable thickness for the active layer 22 includes, but isnot limited to, a thickness less than 50 nanometers, 10 nanometers, 5nanometers, or 1 nanometer.

During organic fuel generation, the oxygen can be reduced at thecathode. The oxygen reduction is a parasitic reaction that compromisesthe fuel generation. The active layer can have a thickness selected suchthat the active layer reduces access of oxygen in the electrolyte to thesurface of the electrode base. As a result, the active layer can act asa barrier to oxygen reduction in that the oxygen cannot pass through thebarrier to contact the electrode base. An active layer thickness thatallows the active layer to reduce or prevent oxygen reduction at thecathode includes, but is not limited to, an active layer thicknessgreater than greater than 0.5 nanometer, 1 nanometer, 10 nanometers, 100nanometers.

During use of the electrode for CO₂ reduction, the electrode istypically exposed to an acidic electrolyte. For instance, an electrolyteused in CO₂ reduction can have a pH less than 7, 5, or even 3 althoughhigher pH levels are possible. Prior electrodes have not been tolerantof these pH levels; however, the active layer 22 has proven to be highlytolerant of acidic electrolytes and has accordingly extended the workinglife of these electrodes. For instance, in some instances, the activelayer can be thick enough to act as a barrier between the electrolyteand the electrode base in that the electrolyte cannot pass through thebarrier to contact the electrode base. Increasing the thickness of theactive layer 22 has provided increased electrode protection. An activelayer 22 thickness that can provide protection from an acidicelectrolyte includes, but is not limited to, a thickness greater than0.5 nanometer, 1 nanometer, 10 nanometers, 100 nanometers. Accordingly,when tunneling is desired in the active layer 22 and the electrode isexposed to an acidic electrolyte, the active layer 22 can have athickness greater than 0.5 nanometer, 1 nanometer, 10 nanometers,nanometers and/or less than less than 50 nanometers, 10 nanometers, 5nanometers, or 1 nanometer.

The electrodes illustrated in FIG. 2 through FIG. 3B can serve as thecathode in the electrolysis system disclosed in the context of FIG. 1.In these instances, an electrolyte can be included in the active layerand/or in the electrolyte. Additionally or alternately, an activator canbe included in the active layer and/or in the electrolyte. FIG. 1illustrates the activator and intermediate being present in theelectrolyte. However, when the activator is included in an electrodeconstructed according to FIG. 2 through FIG. 3B, the intermediate isformed in the active medium and the activator is released in the activemedium in response to reduction of CO₂ in the intermediate. Accordingly,there need not be a substantial concentration of the activator and/orintermediate in the electrolyte.

The polymeric medium can include, consist or, or consist essentially ofone or more polymers. In some instances, the one or more polymers areamorphous. Suitable polymers for inclusion in the polymeric mediumsinclude polymers with organic backbones, silane backbones, or siloxanebackbones. For instance, the polymer backbone can include one, two,three, or four components selected from the group consisting of carbon,oxygen, nitrogen, and silicon. The repeating units in the backbone canbe saturated or unsaturated. In some instances the repeating units inthe backbone are unsaturated. The repeating units in the backbone can belinear or cyclic or can include linear and/or cyclic segments. Examplesof the polymer include polymers having a backbone that includes orconsists of a poly(alkylene), a poly(alkylene oxide), apoly(alkenylene), poly(alkenylene oxide), a poly(carbonate),poly(heteroalkylene), and poly(hydrocarbylene). The backbone can be acopolymer that has a backbone that includes at least two differentrepeating units selected from the group consisting of alkylenes,alkylene oxides, alkenylenes, alkenylene oxides, and carbonates. Thecopolymer can be alternating, statistical, random, periodic, or block.In some instances, the polymer backbone is not saturated. Accordingly,all or a portion of the repeating units in the backbone can includedouble bonds. For instance, all or a portion of the repeating units inthe polymer can include one or more carbon-carbon double bonds.

The polymer is preferably cross linked in that at least a portion of therepeating units are bonded to a cross-linker that is linked to at leasttwo different backbones although a portion of the cross-linkers can belinked to the same backbone in two or more different locations. In someinstances, the cross-linkers are covalently bonded to at least twodifferent backbones. Suitable cross linkers are organic cross-linkers.For instance, the cross-linkers can include or consist of carbon and oneor more components selected from the group consisting of oxygen,nitrogen, hydrogen, and halogens. In some instances, the cross linkerincludes or consists of alkylenes, alkylene oxides, alkenylenes,alkenylene oxides, bivalent ethers, and bivalent carbonates. Forinstance, the cross linker can include or consist of poly(alkylene)s,poly(alkylene oxide)s, a poly(alkenylene)s, poly(alkenylene oxide)s,poly(carbonate)s, and polysiloxanes. These cross-linkers can be linearor cyclic or can include linear segments and cyclic segments.Additionally or alternately, these cross-linkers can be substituted orunsubstituted. Further, these cross-linkers can be fully or partiallyhalogenated or can exclude halogens. In some instances, the cross-linkerincludes or consists of a reaction component. For instance, across-linker can include an organometallic CO₂ reduction catalyst, or anactivator. In some instances, the polymer becomes more rigid as thepercentage of the backbone atoms that are linked to cross-linkersincreases. In some instances, more than 1%, 10%, or 50% of the atoms inthe backbone of the polymer are linked to a cross-linker.

The atoms in a backbone can also be linked to one or more sidechainsthat are not cross-linkers in that they are linked to only one backbone.Suitable sidechains include or consist of organic sidechains and/ororganometallic sidechains. For instance, the sidechains can include orconsist of carbon and one or more components selected from the groupconsisting of oxygen, nitrogen, hydrogen, halogens, and metals. Themetals included in the sidechains can be selected from the groupconsisting of Fe, Co, Ni, Cu, Ag, Au, Sn, Mo, Ir, Pt, Ru, Ti, Zr, Ta,Mg, Li, Hg, Al and Zn. In some instances, the sidechains includes orconsists of alkylenes, alkylene oxides, alkenylenes, alkenylene oxides,bivalent ethers, and bivalent carbonates. For instance, a sidechain caninclude or consist of poly(alkylene)s, poly(alkylene oxide)s, apoly(alkenylene)s, poly(alkenylene oxide)s, poly(carbonate)s, andpolysiloxanes. These sidechains can be linear or cyclic or can includelinear segments and cyclic segments. Additionally or alternately, thesesidechains can be substituted or unsubstituted. Further, thesesidechains can be fully or partially halogenated or can excludehalogens. In some instances, the sidechains include or consist of areaction component. For instance, the sidechains can include or consistof an organometallic CO₂ reduction catalyst, and/or an activator.

The backbone atoms that are not linked to sidechains or cross-linkerscan be linked to hydrogens, halogens, or terminal groups.

One example of the polymer can be represented by the following FormulaI:

wherein m is greater than or equal to 0; n is greater than or equal to0; p is greater than or equal to 0; q is greater than or equal to 0,S/C¹ represents a sidechain that includes or consists of a catalyst,S/C² represents a sidechain that includes of consists of an activator;C/L represents a cross-linker that is bonded to the backbone of anotherpolymer chain represented by Formula I or to another location onbackbone represented by Formula I; R¹, R², R³, R⁴, R⁵, R⁶, R⁷ eachrepresents nil, an alkylene, alkylene oxide, alkenylene, alkenyleneoxide, bivalent ether, bivalent carbonate, alkyl, aryl, amine, ester,silane, and can all be the same, can all be different, or some can bethe same and some different; R⁸, R⁹, and R¹⁰, each represent S/C¹, S/C²,C/L, a hydrogen, halogen, and can all be the same, can all be different,or some can be the same and some different. S/C¹, S/C², C/L can each bean organic moiety. For instance, S/C¹, S/C², C/L can include analkylene, alkylene oxide, alkenylene, alkenylene oxide, bivalent ether,bivalent carbonate, alkyl, aryl, amine, ester, silane. C/L can includeor consist of an alkylene, alkylene oxide, alkenylene, alkenylene oxide,bivalent ether, bivalent carbonate, alkyl, aryl, amine, ester, silane.

In some instances, m, n, and p are each greater than 0. In someinstances, m or p is 0. Suitable ratios of m+n+q: p include, but are notlimited to, ratios greater than 0.2, 0.5, 1, or 2. One or more of theterminal groups for the polymer chain can also be S/C¹, S/C² or C/L.When any of R₁-R₁₀ represents an organic moiety, all or a portion ofR₁-R₁₀ can be linear or cyclic or can include linear and/or cyclicsegments. Additionally or alternately, when R₁-R₁₀ represents an organicmoiety, all or a portion of R₁-R₁₀ can be substituted or unsubstitutedand/or can be fully or partially halogenated or can exclude halogens.Although a strict interpretation of Formula I results in a blockcopolymer, Formula I can represent an alternating, statistical, random,periodic, or block. The repeating units in Formula may each represent amonomer residual but do not need to represent a monomer residual. Insome instances, m+n+p is greater than 10, 100, or 1000 and/or less than10000, 1000, 100. Additionally or alternately, the ratio of m:n or m+p:ncan be greater than 1, 10, 100 and/or less than 1, 0.1, 0.01 The polymerrepresented by Formula I can include repeating units in addition to therepeating units illustrated in Formula I or can be limited to therepeating units illustrated in Formula I.

An example of a polymer according to Formula I is provided in thefollowing Formula II:

where r is greater than or equal to 0. In this polymer, S/C¹ includesferrocene as a catalyst, S/C² includes an N-heterocyclic carbene as anactivator, and C/L is a cross-linker where b/b represents the backboneof another polymer chain represented by Formula I or II or representsanother location on backbone represented by Formula II. Although theterminal groups are not illustrated, one or more of the terminal groupscan each be S/C¹, S/C², or C/L. The backbone is unsaturated and therepeating units include alkenylenes. Formula II illustrates that thealkenylenes disclosed above can have one or more olefin groups.

The polymer can be generated using common polymer synthesis technologiessuch as olefin metathesis. When the polymer is generated by olefinmetathesis, the sidechains (S/C¹ and S/C²) can be added by crossmetathesis. For instance, the reactive components to be included in thepolymer can include a terminal olefin, can be included in a compoundwith a terminal olefin or can be modified to include a tether with aterminal olefin. The resulting compounds operate as sidechain precursorsthat cross metathesize with olefins on the polymer. As an example of asidechain precursor, consider that ferrocene is disclosed above as acatalyst. Vinyl ferrocene includes the ferrocene with a tether having aterminal olefin and can accordingly serve as a sidechain precursor for asidechain that includes ferrocene as a catalyst. FIG. 4A illustratesanother example of a catalyst with a tether having a terminal olefinthat can serve as a sidechain precursor for a sidechain that includes acatalyst. As an example for a sidechain that includes an activator, notethat the N-heterocyclic carbene represented by

has been disclosed as an activator. The compound represented byrepresented by

includes an embodiment of this N-heterocyclic carbene with a tetherhaving a terminal olefin. Accordingly, this compound can serve as asidechain precursor for a sidechain that includes an N-heterocycliccarbene as an activator. FIG. 4B illustrates another example of anactivator with a tether having a terminal olefin that can serve as asidechain precursor for a sidechain that includes an activator. Avariety of organic compounds that include two or more olefins can serveas cross-linker precursors. The cross-linker precursors can include allor a portion of the cross-linker that is included in the polymer. Anexample of a suitable cross-linker precursor includes, but is notlimited to, 5-ethylidene-2-norbornene.

FIG. 5 illustrates the polymer of Formula II generated using a ringopening metathesis polymerization (ROMP). Cyclooctadiene and ethyl vinylether are reacted in the presence of an olefin metathesis catalyst toform a first monomer for the polymer. The reaction is a ring openingreaction where the ethyl vinyl ether serves as a quencher. In a secondreaction, 5-ethylidene-2-norbornene serves as a second monomer. Thefirst monomer and the second monomer are polymerized in the presence ofa second metathesis catalyst. The polymerization occurs in the presenceof vinyl ferrocene sidechain precursor and the N-heterocyclic carbenesidechain precursor activator represented by

The terminal olefins in these sidechain precursors cross metathesizewith an olefin in the 5-ethylidene-2-norbornene to form the sidechainsillustrated in the Formula II polymer. Additionally, an olefin an5-ethylidene-2-norbornene that is part of the polymer can crossmetathesize with a 5-ethylidene-2-norbornene included in a secondpolymer chain to form the cross-linker.

Suitable catalysts for the metathesis polymerization and the ringopening metathesis polymerization (ROMP) include, but are not limitedto, metathesis catalysts. Examples of these catalysts include, but arenot limited to, the Grubbs catalysts and ruthenium complexes.

Although the polymer for the active layer is disclosed as having the oneor more reaction components each included in a side chain, the one ormore reaction components can be included in a cross-linker and/or in thebackbone.

Although the polymer for the active layer is disclosed in the context ofa polymer that includes catalyst and activator, the polymer can excludethe catalyst and/or the activator. For instance, the polymer can includethe catalyst and the activator can be present at another location in thesystem such as the electrolyte. Alternately, the polymer can include theactivator and the catalyst can be present at another location in thesystem.

As noted above, the active layer 22 of the electrode includes, consistsof, or consists essentially of one or more of the polymers. The one ormore polymers can be generated directly on the electrode base 24 inorder to immobilize the polymeric medium on the electrode base 24 or toattach the polymeric medium to the electrode based. When the activelayer 22 is to be patterned, the polymerization can occur through aphotomask and the polymerization catalyst can be photoactivatable. Apositive photoactivatable metathesis catalysts catalyze metathesis whenactivated by light but do not provide substantial catalytic activitywhen dark and a negative photoactivatable metathesis catalyst wouldcatalyze metathesis when dark but not when light is incident on thecatalyst. An example of a positive photoactivatable metathesis catalystis represented by

When a polymerization reaction is carried out in a medium having apositive photoactivatable metathesis catalysts, the polymer is formed inthe portions of the medium that are illuminated with an appropriatelight source but are not substantially formed in the dark portions ofthe medium. Accordingly, when the medium is illuminated through thephotomask, the pattern of the photomask is transferred to the mediumafter the unpolymerized portion of the medium is removed (development).As a result, a patterned polymeric medium can be generated by generatingthe polymer in the presence of a photoactivatable catalyst that isilluminated through a photomask. This methodology can be employed togenerate an active layer having features smaller than 100 microns, 10microns or 1 micron. Accordingly, the active layer 22 on an electrodecan have features with dimensions smaller than 100 microns, 10 micronsor 1 micron. Examples of these dimensions include, but are not limitedto, horizontal or lateral dimensions when the active layer is sitting ona flat and horizontal surface such as diameters, widths, lengths.

The above discussion of the CO₂ reduction catalyst is largely directedto the use of molecular catalysts; however, nanoparticles can also serveas CO₂ reduction catalyst. For instance, nanoparticles that include Fe,Co, Ni, Cu, Ag, Au, Sn, Mo, Ir, Pt, Ru, Ti, Zr, Ta, Mg, Li, Hg, Al andZn can serve as a CO₂ reduction catalysts. Nanoparticles are particlesbetween 1 and 1000 nanometers in size. In some instances, thenanoparticles are inorganic. The nanoparticles can be included in theactive layer. For instance, the nanoparticles can be covalently bondeddirectly to one or more polymers within the polymeric medium using knownpolymer generation methods and known interfacial chemistries. Forexample, ligands bound to the nanoparticle surfaces can be crosslinkedinto a polymer matrix. Alternately, the nanoparticles can be entrappedwithin the polymeric medium without being covalently bonded to any ofthe polymers included within the polymeric medium. For instance, thenanoparticles can be located in pores within the polymeric medium.

The electrode can be incorporated into other devices such as a solarfuels generator. FIG. 6A is a cross section of a solar fuels generator.FIG. 6B is a sideview of the solar fuels generator shown in FIG. 6Ataken looking in the direction of the arrow labeled V in FIG. 6A. Thecross section shown in FIG. 6A can be taken along the line labeled A inFIG. 6B.

The solar fuels generator includes a barrier 62 between a first phase 64and a second phase 66. The barrier 62 includes or consists of one ormore potential generation components 68 and one or more separatorcomponents 70. FIG. 6A illustrates the potential generation components68 linked with separator components 70 so as to form the barrier 62between the first phase 64 and the second phase 66. The potentialgeneration components 68 are alternated with the separator components70. Each potential generation component 68 contacts both the first phase64 and the second phase 66 and each separator component 70 contacts boththe first phase 64 and the second phase 66. The barrier 62 is formedsuch that the first phase 64 can be maintained at a different chemicalcomposition than the second phase 66. For instance, the barrier 62 canbe impermeable or substantially impermeable to nonionic atoms and/ornonionic compounds.

The potential generation components 68 include anodes 72 and cathodes74. As illustrated by the arrow labeled L_(A) and L_(C), light isincident on the anodes 72 and/or cathodes 74 during operation of thesolar fuels generator. The anodes 72 and cathodes 74 convert thereceived light into excited electron-hole pairs that drive a chemicalreaction such as electrolysis of water. The anodes 72 include an anodelight absorber 76 selected to absorb light at a wavelength to which theanodes 72 will be exposed during operation of the solar fuels generator.Additionally, the cathodes 74 include a cathode light absorber 78selected to absorb light at a wavelength to which the cathodes will beexposed during operation of the solar fuels generator.

Suitable materials for the anode light absorbers 76 and the cathodelight absorbers 78 include, but are not limited to, semiconductors. Insome instances, the anode light absorbers 76 include or consist of asemiconductor and/or the cathode light absorbers 78 include or consistof a semiconductor. The bandgap of the semiconductors included in theanode light absorbers 76 can be larger than the bandgap of thesemiconductors included in the cathode light absorbers 78. Suitablesemiconductors for the anode light absorbers 76 include, but are notlimited to, metal oxides, oxynitrides, sulfides, and phosphides that arestable in an oxidizing environment such as WO₃, TiO₂, and TaON. Suitablesemiconductors for the cathode light absorbers 78 include, but are notlimited to,p-type silicon, InP, Cu₂O, GaP, and WSe₂.

In some instances, the anode light absorbers 76 and/or the cathode lightabsorbers 78 are doped. The doping can be done to form one or more pnjunctions within the anode light absorbers 76 and the cathode lightabsorbers 78. For instance, the anode light absorber 76 can be an n-typesemiconductor while the cathode light absorber 78 can be a p-typesemiconductor. A pn junction can also be present within either thecathode light absorbers 78 or the anode light absorber 76 or both, andis arranged so that electrons flow from the cathode light absorber 78 toa reduction catalyst (discussed below) and holes flow from the anodelight absorber 76 to an oxidation catalyst (discussed below).

The dashed lines at the interface of the anode light absorber 76 and thecathode light absorber 78 illustrate an interface between the materialsof the anode light absorber 76 and the cathode light absorber 78.However, the anode light absorber 76 and the cathode light absorber 78can be the same material and/or include the same dopant. As a result, ananode light absorber 76 and the interfaced cathode light absorber 78 canbe a continuous block of material. In these instances, the dashed linesshown in FIG. 6A may represent a feature that is not discernible in thesolar fuels generator. One example of a material that can serve as boththe anode light absorber 76 and the cathode light absorber 78 is p-typesilicon, which can function as the absorber on both the anode andcathode. In particular, p-type silicon is a candidate for the cathodematerial because it is cathodically stable under illumination in acidicaqueous media and in conjunction with various metal catalysts can evolveH₂(g) from H₂O.

Other possible choices for the light anode light absorber 76 and/or thecathode light absorber 78 include semiconductors having wider bandgapsthan silicon that are stable in a water vapor medium such as oxidesemiconductors. Some of the oxide semiconductors that can be used as alight absorber include, but are not limited to: tandem structure anodes,including tungsten oxide (WO₃), bismuth vanadium oxide (BiVO₄),tantalumoxynitride (TaON), and titanium oxide (TiO₂); tandem structurecathodes, including silicon (Si), cuprous oxide (Cu2O), galliumphosphide (GaP), gallium arsenide (GaAs), and indium phosphide (InP);single material electrodes, including strontium titanate (SrTiO₃),strontium niobate (SrNbO₃), and titanium oxide (TiO₂); multijunctionphotovoltaics, including triple junction amorphous silicon (a-Si), andvertically stacked epitaxially grown III-V semiconductors with tunneljunctions; and series connected photovoltaics, including silicon (Si)cells, gallium arsenide (GaAs) cells, cadmium telluride (CdTe) cells,and Copper Indium Gallium Selenide (CIGS) thin film cells.

The absorption of light by the cathode light absorber 78 and the anodelight absorber 76 generates the photovoltage that drive the CO₂reduction and effectively acts as the voltage source of FIG. 1. Whensemiconductors are used for the cathode light absorber 78 and the anodelight absorber 76, the achievable voltage depends on the choice ofsemiconductor materials, the associated bandgaps, and dopingarrangements as is known in the solar cell arts. Accordingly, thematerial selections and arrangements can be selected to provide thedesired voltage levels. For instance, tandem and multijunctionstructures in which two or more semiconductors in series add theirvoltages together can be used in order to achieve elevated voltages.

The anodes 72 include one or more oxidation catalyst layers 86 that eachinclude or consist of one or more oxidation catalysts. One or moreoxidation catalyst layers 86 can be in direct physical contact with theanode light absorber 76. As is evident from FIG. 6A, when light is to beincident on the anode, the light passes through one or more oxidationcatalyst layers 86 before reaching the anode light absorber 76. As aresult, the one or more oxidation catalyst layers 86 can be transparentand/or thin enough that the one or more oxidation catalyst layers 86 donot absorb an undesirably high level of the incoming light. A suitablethickness for an oxidation catalyst layer 86 includes, but is notlimited to, a thickness less than 10 nm to a few micrometers.

The oxidation catalyst layer 86 and/or the oxidation catalyst caninclude, consist of, or consist essentially of a metal oxide thatincludes or consists of oxygen, cerium, and one or more second metals.In one example, the oxidation catalyst includes a metal oxiderepresented by (La_(v)Ni_(w)Co_(y)Ce_(z))O_(x) where v+w+y+z=1, v isgreater than or equal to 0 and less than 1, w is greater than or equalto 0 and less than 1, y is greater than or equal to 0 and less than 1,and z is greater than or equal to 0 or greater than or equal to 0.1and/or less than 1 or less than or equal to 0.8, x is greater than 0 orgreater than or equal to 0.5 and/or less than or equal to 3, 2, or 1.6,and at least one, two, three or four of v, w, y, and z is greater than0. As an example, (La_(0.1)Ni_(0.1)Co_(0.3)Ce_(0.5))O_(x) is suitablefor use as the oxidation catalyst.

The cathodes 74 include one or more reduction layers 88. The activelayer 22 disclosed above can serve as the reduction layer reductionlayer 88 and the cathode light absorber 78 can serve as the electrodebase 24 disclosed above. Accordingly, one or more of the reductionlayers 88 can include, consist of, or consist essentially of thepolymeric medium disclosed above and the polymeric medium can include,consist of, or consist essentially of one or more polymers thatconstrain one or more of the reaction components. Accordingly, the oneor more reduction layers 88 include one or more of the catalysts and/orone or more of the activators.

When light is to be incident on the cathode, the light passes throughone or more reduction layers 88 before reaching the cathode lightabsorber 78. As a result, the one or more reduction layers 88 can betransparent and/or thin enough that the one or more reduction layers 88do not absorb an undesirably high level of the incoming light. Asuitable thickness for a reduction layer 88 includes, but is not limitedto, a thickness of about 1 nm, or 5 nm to 5 μm. In some instances, thecatalyst layer 88 does not need to be transparent. For instance, thecatalyst layer 88 need not be transparent if it is facing down (i.e., isaway from the incident light source).

The one or more reduction layers 88 are positioned on a surface of thecathode light absorber 78 such that a line that is perpendicular to thesurface extends from the surface through one or more of the reductionlayers 88 before extending through the second phase 66. The one or morereduction catalyst layers can be positioned such that the one or morereduction catalyst layers are on more than 10%, 30%, 50%, 75%, or 90% ofthe surface of the cathode light absorber 78.

The separator components 70 include or consist of a separator 90 locatedbetween the first phase 64 and the second phase 66. The separator 90 isionically conductive. In some instances, the separator 90 iscationically conductive while concurrently being sufficientlynonconductive to the other components of the first phase 64 and thesecond phase 66 that the first phase 64 and the second phase 66 remainseparated from one another. For instance, in some instances, theseparator 90 is cationically conductive and non-conductive orsubstantially non-conductive to nonionic atoms and/or nonioniccompounds. In some instances, the separator 90 is cationicallyconductive while being non-conductive or substantially non-conductive tononionic atoms and/or nonionic compounds and also to anions.Accordingly, the separator 90 can provide a pathway along which cationscan travel from the first phase 64 to the second phase 66 withoutproviding a pathway or a substantial pathway from the first phase 64 tothe second phase 66 to one, two, or three entities selected from a groupconsisting of anions, nonionic atoms or nonionic compounds. In someinstances, it may be desirable for the separator 90 to conduct bothanions and cations. For instance, when the first phase 64 and/or thesecond phase 66 has elevated pH levels a separator 90 that conducts bothanions and cations may be used. As a result, in some instances, theseparator 90 conducts cations and anions but not nonionic atoms ornonionic compounds.

Additionally, the separator 90 should be able to exchange ionssufficiently to maintain a desired pH gradient and separate the reactionproducts sufficiently to prevent them from re-combining. A suitableseparator 90 can be a single layer or material or multiple layers ofmaterial. Suitable materials for the separator 90 include, but are notlimited to, ionomers and mixtures of ionomers. Ionomers are polymersthat include electrically neutral repeating units and ionized repeatingunits. Suitable ionomers include copolymers of a substituted orunsubstituted alkylene and an acid such as sulfonic acid. In oneexample, the ionomer is a copolymer of tetrafluoroethylene andperfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid. A suitable materialhaving a structure according to Formula I is sold under the trademarkNAFION®. NAFION® is an example of a material that is cationicallyconductive of cations but is not conductive of anions or nonionic atomsor nonionic compounds. Another suitable separator 90 includes NAFION®functionalized with one or more components selected from a groupconsisting of dimethylpiperazinium cationic groups, glass frits,asbestos fibers, block copolymer formulated layers, and poly(aryleneether sulfone) with quaternary ammonium groups.

During operation, the solar fuels generator is exposed to light such assunlight, terrestrial solar illumination, AM1 solar radiation, orsimilar illumination having approximately 1 kilowatt per square meter ofincident energy or less. These light sources can be unconcentrated orcan be concentrated using known light concentration devices andtechniques. In some instances, the solar fuels generator is orientedsuch that the light travels through the anodes before reaching thecathodes. When the anode light absorber 76 has a larger bandgap than thecathode light absorber 78, the anodes absorb higher energy (shorterwavelength) light and allow lower energy (longer wavelength) light topass through to the cathodes. The cathodes can then absorb the longerwavelengths. Alternately, the light can be incident on both the anodesand the cathodes or can be incident on the cathodes before reaching theanodes.

The absorption of light by an anode light absorber 76 generateshole-electron pairs within the anode light absorber 76. The presence ofan n-type anode light absorber 76 in the first phase 64 produces anelectrical field that causes the holes to move to the surface of theanode light absorber 76 and then the surface of the oxidation catalystlayer 86 where the oxidation of water occurs as illustrated by the firstreaction in FIG. 6A. The electrons generated in the anode light absorber76 move toward the cathode light absorber 78 as a result of theelectrical field.

The protons generated in the first reaction move from the oxidationcatalyst layer 86 into the first phase 64. Since the separator 90 iscationically conductive, the protons move from the first phase 64 to thesecond phase 66 through the separator 9. A suitable thickness for theseparator 90 is a thickness of about 100 nm to 1 μm or more. The secondphase includes the CO₂ that is to be reduced. For instance, the firstphase could include or consist of an electrolyte disclosed in thecontext of the electrolyte system or can have a different composition.The CO₂ and the protons in the second phase travel to the reductionlayers 88.

The absorption of light by the cathode light absorber 78 generateshole-electron pairs within the cathode light absorber 78. The presenceof a p-type cathode light absorber 78 in the second phase 66 produces anelectrical field that causes the electrons within the cathode lightabsorber 78 to move to the surface of the cathode light absorber 78 andthen into the reduction layers 88. As noted above, the one or morereduction layers 88 can include the one or more activators. As a result,the CO₂ that travels to the one or more reduction layers 88 can interactwith the one or more activators so as to form an intermediate such as aCO₂ adduct. The electrons, protons, and intermediate react so as togenerate at least the organic fuel and the activator. Although FIG. 6Aillustrates methane as the generated fuel, the generation of other fuelsis possible depending on the selection of the activator. The holesgenerated in the cathode light absorber 78 by the absorption of lightmove from the cathode light absorber 78 toward the anode light absorber76 as a result of the electrical field and can recombine with theelectrons from the anode light absorber 76. When the generated fuel is agas, the generated fuel can bubble up through a liquid second phase andleave the second phase.

The first phase 64 is generally different from the second phase 66. Forinstance, the first phase 64 generally has a different chemicalcomposition than the second phase 66. The first phase 64 and the secondphase can both be a liquid. For instance, the first phase 64 can be astanding, ionically conductive liquid such as water.

The one or more oxidation catalyst layers 86 illustrated in FIG. 6A caninclude materials in addition to the oxidation catalyst. For instance,an oxidation catalyst layer 86 can include one or more componentsselected from a group consisting of electrically conductive fillers,electrically conductive materials, diluents, and/or binders.

A suitable method for forming oxidation catalyst layers 86 on the anodelight absorber 76 includes, but is not limited to, electrodeposition,sputtering, electroless deposition, spray pyrolysis, and atomic layerdeposition. Alternately, the catalyst layer 86 can be a catalytic layerformed directly on the anode light absorber 76 as described below. Asuitable method for forming reduction layers 88 on the cathode lightabsorber 78 includes, but is not limited to, polymerizing the polymericlayer directly on the cathode light absorber 78. A suitable method forattaching the separator 90 to the anodes 72 and/or cathodes 74 includes,but is not limited to, clamping, lamination, sealing with epoxy or glueand the like.

EXAMPLES Example 1

Controlled potential electrolysis (CPE) experiments were performed usingan electrolysis device with a working chamber volume of 40 mL and acounter chamber volume of 20 mL within a total cell volume of 188.5 mL.Different electrolytes were prepared for different experiments. Theelectrolyte was prepared with a solvent having 0.2 M nBu₄NBF₄ solutionin 60 mL of a 4:2 (v:v) methylene chloride/trifluoroethanol mixture. Thesolvent was saturated with CO₂. Some electrolytes included 466 mg of the1,3-bis(2,6-diisopropylphenyl)imidazolium carboxylate (MW: 432.5 g/mol)serving as the intermediate. Some electrolytes included 3.4 mgNiCyclamCl₂ (MW: 320 g/mol) as a catalyst.

The electrolysis was performed at −1.5 V (vs Ag/AgNO₃) for two hours.After two hours, 10 mL of the headspace volume were sampled analyzed byan Agilent GC-TCD instrument. Faradaic efficiencies were calculatedassuming an 8e⁻ transformation per mol of methane detected, a 2e⁻transformation/H₂ detected and a 2e⁻ transformation/mol CO detected. Theresults are presented above in Table 1.

Example 2

Cyclic voltammograms were prepared for several different electrolytes.The cyclic voltammograms were collected at a glassy carbon workingelectrode (3 mm diameter, BASi) with a C rod counter electrode (99.999%,Strem). The cyclic voltammograms were recorded after rigorous exclusionof air via nitrogen purge. Data workup was performed on OriginProv8.0988.

Several electrolytes were prepared in 5 mL CH₂Cl₂ (0.1 M nBu₄NBF₄) Eachelectrolyte included 0.5 mL trifluoroethanol as a proton source and 1.2mM [Ni (cyclam)]Cl₂, as a catalyst. The first electrolyte excluded CO₂.The second electrolyte included CO₂ and an intermediate. The thirdelectrolyte included the intermediate1,3-bis(2,6-diisopropylphenyl)imidazolium carboxylate (MW: 432.5 g/mol)and excluded CO₂. The fourth electrolyte included the intermediate1,3-bis(2,6-diisopropylphenyl)imidazolium carboxylate (MW: 432.5 g/mol)and included CO₂.

The cyclic voltammograms that resulted from each of the differentelectrolytes are presented in FIG. 7. The first electrolyte showedlittle reduction current at electrode potentials positive of ca. −2.0 V.The addition CO₂ in the second electrolyte resulted in enhanced cathodiccurrent at potentials negative of ca. −1.3V. In contrast, the presenceof the intermediate in the third and fourth electrolyte yielded enhancedcathodic currents and distinct waves with peak potentials ranging from−1.5 to1.7V vs (Ag/AgNO₂). The enhanced currents at more positiveelectrode potentials observed when the NHC—CO₂ carboxylate was presentsuggested a CO₂ reduction pathway different from what is observed withunreacted CO₂, and this hypothesis is confirmed by the Example 1results.

Example 3

A cross-linked polymer was generated according to Formula with S/C¹being a sidechain that included a catalyst according to FIG. 4A. Thebis-pyridine olefin metathesis catalyst illustrated in FIG. 5 (1.3 mg)was placed under argon and dissolving in 2 mL dichloromethane. To thiscatalyst solution was quickly added 1.5 mL 1,5-cyclooctadiene, thesolution became a semi-solid in 10 seconds and was allowed to react for1 minute before quenching with 3 mL ethyl vinyl ether. The viscoussolution was slowly stirred for 5 minutes, sealed under argon, andsonicated for 1 hour. The volatiles were removed on a rotary evaporator,to yield semisolid poly(COD), colored light yellow by the quenchedcatalyst (the photoactive vinyl ether complex). Ethylidene norbornene(10 mL) was added to this mixture, which was cooled to 0° C. andsonicated for 1 hour. The partially dissolved mixture was placed on anice bath and stirred until fully dissolved, while allowing the bath towarm to room temperature. The result is a light yellow, viscous solutionweighing approximately 10 grams (“First Solution”).

The rhenium complex illustrated in FIG. 4A (3.0 mg) was added to a smallvial and dissolved in 1.5 mL acetonitrile. A 100 microliter aliquot ofthis solution was added to 1 mL 5-ethylidene-2-norbornene, along with100 microliters of the “First Solution.” The result was spun at 7 k RPMfor 60sec and then irradiated for 2 minutes @ 254 nanometers. The resultwas rinsed in toluene to check for cross-linking.

Although the electrode and active layer are disclosed in the context ofCO₂ reduction, the electrode and active layer can be used in othercontexts. For instance, the polymer in the active layer can includecatalysts for redox reactions other than CO₂ reduction and/or activatorsfor reactions other than CO₂ reduction. As an example, the polymer inthe active layer can include catalysts for olefin metathesis.

Other embodiments, combinations and modifications of this invention willoccur readily to those of ordinary skill in the art in view of theseteachings. Therefore, this invention is to be limited only by thefollowing claims, which include all such embodiments and modificationswhen viewed in conjunction with the above specification and accompanyingdrawings.

What is claimed is:
 1. A method of CO₂ reduction, comprising: bondingCO₂ with an amine activator so as to form an intermediate; and applyingan electrical potential to the intermediate under conditions that causereduction of the CO₂ bound in the intermediate.
 2. The method of claim1, wherein reduction of the CO₂ in the intermediate generates a liquidorganic fuel.
 3. The method of claim 1, wherein reduction of the CO₂ inthe intermediate generates a gaseous organic fuel.
 4. The method ofclaim 3, wherein the reduction of the CO₂ generates methane.
 5. Themethod of claim 1, wherein the CO₂ is linear before bonding with theactivator but has a bent configuration in the intermediate.
 6. Themethod of claim 1, wherein the intermediate is a carbamate or carbamicacid.
 7. The method of claim 6, wherein the intermediate is N-carbamateamine-CO₂.
 8. The method of claim 1, wherein the activator is releasedfrom the intermediate upon reduction of the CO₂ in the intermediate. 9.The method of claim 1, wherein the CO₂ is bonded to the activator in aCO₂ scrubber.
 10. A CO₂ reduction device, comprising: an amine activatorthat bonds with CO₂ so as to form an intermediate; and electrodes thatapply an electrical potential to the intermediate under conditions thatcause reduction of the CO₂ in the intermediate.
 11. The device of claim10, wherein reduction of the CO₂ in the intermediate generates methane.12. The device of claim 10, wherein the CO₂ is linear before bondingwith the activator and has a bent configuration in the intermediate.